Semiconductor device and its manufacturing method

ABSTRACT

A semiconductor device includes a semiconductor substrate; a gate oxide film made on the semiconductor substrate; and first transistors each having a first gate formed on the gate oxide film and a pair of source/drain formed in confrontation in the semiconductor substrate. The gate oxide film has a higher nitrogen concentration in its portion nearer to the first gates than that of its portion nearer to the semiconductor substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a semiconductor device and itsmanufacturing method, and more particularly, to those characterized inits process for making a gate oxide.

[0003] 2. Related Background Art

[0004] NO or N₂O gas is typically used as the source material gas formaking a nitrogen oxide film as a process for manufacturing asemiconductor. In this case, a maximum peak of nitrogen concentrationdistribution is formed near a Si substrate.

[0005] However, since nitrogen has a fixed charge, there occurs theproblem that, if the nitrogen concentration is too high, in the Si side,it causes transistors fluctuate in threshold value, and causes adecrease of the channel current.

[0006] Taking it into consideration, for the purpose of locating thepeak of the nitrogen concentration away from a boundary face of the Sisubstrate, re-oxidation is required normally after the nitrogen oxidefilm is formed. In this case, however, a decrease in nitrogen quantityin the film due to the re-oxidation or non-uniform re-oxidation maycause deterioration of the gate reliability.

[0007] That is, it was difficult to arbitrarily control the nitrogenconcentration and its distribution with conventional manufacturingmethods of semiconductor devices.

[0008] On the other hand, upon making a gate oxide film of a flushmemory, a nitrogen oxide film made by using NH₃ gas as the sourcematerial gas is often used.

[0009] In case of using NH₃ gas as the source material gas, the nitrogenprofile in the resulting oxide film is characterized in having maximumpeaks of the nitrogen concentration distribution both on the surface ofthe nitrogen oxide film and near the Si substrate. This property iseffective for reducing trapping of electrons and holes in a device likea flush memory configured to injecting electrons from two directions,i.e. from its gate electrode and from the Si substrate.

[0010] However, the use of NH₃ as the source material gas causes theproblem that hydrogen is introduced in addition to nitrogen into thenitrogen oxide film.

[0011] Since hydrogen behaves as an electron trapping site, it must beminimized to increase the reliability of the gate oxide film. For thispurpose, it is necessary to insert re-oxidation after deposition of thenitrogen oxide film. In this case, however, an impurity doped into theSi substrate in a preceding step may spread and fluctuate the thresholdvoltage of the transistors, and may decrease the channel current.Furthermore, nitrogen may spread externally during the additionaloxidation, which may decrease the nitrogen concentration near thesurface of the nitrogen oxide film and may disable to obtain requiredelectric properties.

[0012] That is, even when using NH₃ gas, it was still difficult toarbitrarily control the nitrogen concentration and its distribution withconventional manufacturing methods of semiconductor devices.

[0013] In conclusion, conventional manufacturing methods ofsemiconductor devices involved difficulties in arbitrarily controllingnitrogen concentration and its distribution because of employing theprocess of first fabricating a nitrogen oxide film by using NO, N₂O orNH₃ gas and thereafter re-oxidizing it to make a gate oxide film.

SUMMARY OF THE INVENTION

[0014] It is therefore an object of the invention to provide asemiconductor device and its manufacturing method capable of controllingnitrogen concentration and its distribution by increasing the nitrogenconcentration near its boundary face with an electrode in a process ofmaking a nitrogen oxide film using a gate oxide film.

[0015] According to an aspect of the invention, there is provided asemiconductor device comprising:

[0016] a semiconductor substrate;

[0017] a gate oxide film formed on the semiconductor substrate; and

[0018] first transistors each having a first gate formed on the gateoxide film and a pair of source/drain formed in confrontation in thesemiconductor substrate,

[0019] the gate oxide film having a higher nitrogen concentration inportions thereof nearer to the first gates than that of a portionthereof nearer to the semiconductor substrate.

[0020] In another aspect of the invention, the gate oxide film has ahigher nitrogen concentration in a portion thereof near the first gatethan in a portion thereof near the semiconductor substrate.

[0021] According to a further aspect of the invention, there is provideda method for manufacturing a semiconductor device including firsttransistors each having a first gate on a gate insulating film on asemiconductor substrate, in which the gate insulating film is fabricatedby a process comprising:

[0022] a first step of making a first oxide film on the semiconductorsubstrate;

[0023] a second step of making a silicon film on the first oxide film;

[0024] a third step of making a first nitrogen oxide film on the siliconfilm;

[0025] a fourth step of segregating nitrogen along a surface of thefirst nitrogen oxide film; and

[0026] a fifth step of oxidizing the silicon film to form a secondnitrogen oxide film from the first oxide film, the silicon film and thefirst nitrogen oxide film.

[0027] According to a still further aspect of the invention, there isprovided a method for manufacturing a semiconductor device includingfirst transistors each having a first gate on a gate insulating film ona semiconductor substrate, in which the gate insulating film isfabricated by a process comprising:

[0028] a first step of making a first nitrogen oxide film on thesemiconductor substrate;

[0029] a second step of making a silicon film on the first nitrogenoxide film;

[0030] a third step of making a second nitrogen oxide film on thesilicon film;

[0031] a fourth step of segregating nitrogen along a bottom surface ofthe first nitrogen oxide film and along a top surface of the secondnitrogen oxide film; and

[0032] a fifth step of oxidizing the silicon film to form a thirdnitrogen oxide film from the first nitrogen oxide film, the silicon filmand the second nitrogen oxide film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIGS. 1A through 1D are cross-sectional views of a semiconductordevice under a manufacturing process, which are used to explaining amanufacturing method of a semiconductor device according to the firstembodiment of the invention, following its steps;

[0034]FIG. 2 is a diagram showing a nitrogen profile in a nitrogen oxidefilm made in the first embodiment;

[0035]FIGS. 3A through 3C are cross-sectional views of a semiconductordevice under a manufacturing process, which are used to explaining amanufacturing method of a semiconductor device according to the secondembodiment of the invention, following its steps;

[0036]FIG. 4 is a diagram illustrating a flush-type nonvolatile memoryobtained by the first and second embodiments; and

[0037]FIG. 5 is a diagram showing a nitrogen profile in a nitrogen oxidefilm by the same embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Some embodiments of the invention are explained below withreference to the drawings.

[0039] First Embodiment

[0040]FIGS. 1A through 1D are cross-sectional views of a semiconductordevice under a manufacturing process. They shows a manufacturing methodof a semiconductor device according to the first embodiment of theinvention in the order of its steps.

[0041] As shown in FIG. 1A, following an ordinary process, a thermaloxide film 2, about 60 angstrom thick, is formed as a gate oxide film ona silicon substrate 1, and a poly-silicon film or amorphous siliconfilm, about 20 angstrom thick, is stacked as a silicon film 3 on thethermal oxide film 2 by LP-CVD.

[0042] After that, as shown in FIG. 1B, NO or N₂O gas is introduced asshown by I1 into a chamber controlled to maintain the temperature of900° C. and the pressure of 400 Torr. As a result, the silicon oxidefilm 3 is oxidized, and nitrogen segregates along the surface of anitrogen oxide film 4 stacked thereon. FIG. 1B schematically shows thisstatus.

[0043] Under the condition, the supply of NO, N₂O or O₂ gas iscontinued. As a result, the silicon film 3 is fully oxidized, andfinally, a nitrogen oxide film 5, about 100 angstrom thick, appears onthe silicon substrate 1 as shown in FIG. 1C. FIG. 1C shows this statusschematically.

[0044] At that time, the maximum peak concentration of nitrogen isapproximately 1 through 5 atomic % which is a concentration high enoughto block diffusion of impurities from the gate electrode.

[0045]FIG. 2 shows a nitrogen profile in the nitrogen oxide film made bythe embodiment, and illustrates changes in nitrogen concentration withdistance from the surface of the nitrogen oxide film, combiningrelations among the 20 angstrom thick silicon film 3, 80 angstromthermal oxide film 2 and underlying silicon substrate 1. It will beapparent from FIG. 2 that the maximum peak of the nitrogen concentrationdistribution is located at a position distant by approximately 80angstrom from the silicon substrate 1.

[0046] After that, as shown in FIG. 1D, following an ordinary processflow, a silicon film is stacked on the nitrogen oxide film 5. Then, asillustrated, it is processed into gate electrodes, and BF₂ ision-implanted as shown by I2 for making sources and drains.

[0047] Thereafter, through a further known process, a flush memory, forexample, is obtained as explained below.

[0048] In the embodiment shown here, gate electrodes are doped with B asthe ion-implanted impurity, and the nitrogen oxide film 4 is used as thegate oxide film. Therefore, diffusion of the impurity toward the siliconsubstrate 1 via the gate oxide film does not occur in a later thermalprocess. Additionally, since the nitrogen concentration peak is distantfrom the silicon substrate 1, conventional problems, such as fluctuationof transistors in threshold value and decrease of the channel current,do not occur.

[0049] Although the thermal oxide film 2 used in the instant embodimentis 80 angstrom thick and the silicon film 3 is 20 angstrom thick, thesevalues may be changed to modify the final thickness of the nitrogenoxide film and locate the maximum peak of the nitrogen concentrationprofile at a desired position.

[0050] Although the instant embodiment has been explained as setting thetemperature for making the nitrogen oxide film at 900° C., it may bechanged to any desirable temperature within the range from 600° C. to1000° C. Also regarding the pressure, it is not limited to 400 Torr, butmay be changed to any desirable value within the range from 1 through760 Torr.

[0051] Second Embodiment

[0052] A manufacturing method of a semiconductor device according to thesecond embodiment of the invention is configured similarly to the firstembodiment to first stack a thermal oxide film, about 100 angstromthick, as the gate oxide film on a silicon substrate, and thereaftersputter nitrogen plasma onto the surface of the thermal oxide film whilesupplying 1 SLM of N₂ gas for producing nitrogen from N₂ gas by usinghigh frequency signal, keeping the wafer temperature at 400° C. and thechamber pressure t in 5 Torr. In this case, the maximum peakconcentration of the nitrogen concentration profile is approximately 1through 5 atomic %. Subsequently, annealing is conducted in an O₂atmosphere at 900° C. to bind nitrogen and silicon. Thereafter,similarly to the first embodiment, following an ordinary process flow,the process proceeds with the steps of stacking a silicon film,processing it into gate electrodes and ion-implanting an impurity formaking sources and drains.

[0053] Also the second embodiment, like the first embodiment, canprevent diffusion of the impurity toward the silicon substrate throughthe gate oxide film during a later thermal process, and can preventfluctuation in threshold value of transistors and a decrease of thechannel current, which were conventionally remarked as problems.

[0054] Although the waver temperature used in the instant embodiment is400° C., it may be change appropriately within the range of 350° C. to480° C. The pressure, as well, may be changed as desired within therange from 0.5 to 15 Torr. By appropriately changing these conditions,the final nitrogen concentration and the position of the maximum peak inthe nitrogen concentration profile can be determined as desired.

[0055]FIGS. 3A through 3C are cross-sectional views of a semiconductordevice under a manufacturing process, which are used for explaining amanufacturing method of a semiconductor device according to the secondembodiment of the invention. The second embodiment is directed toapplication of the invention to a manufacturing process of a tunnelingoxide film in a flush memory, for example.

[0056] As shown in FIG. 3A, following an ordinary process, NO or N₂O gasis introduced into a chamber controlled to maintain the temperature of900° C. and the pressure of 400 Torr to stack a 20 angstrom thicknitrogen oxide film 12 on a silicon substrate 11.

[0057] Subsequently, similarly to the first embodiment, a silicon film13 is stacked to approximately 40 angstrom on the nitrogen oxide film 12by LP-CVD.

[0058] After that, NO or N₂O gas is again introduced as shown by I3 intothe chamber held at the temperature of 900° C. and the pressure of 400Torr to oxidize the silicon film 13 and stack a nitrogen oxide film 14.

[0059] As a result, nitrogen segregates along the boundary between thesilicon substrate 11 and the nitrogen oxide film 12, and between thesilicon film 13 and the nitrogen oxide film 14.

[0060] Thereafter, the supply of NO, N₂O or O₂ gas is continued. As aresult, the silicon film 13 is fully oxidized, and finally, a nitrogenoxide film 15, about 100 angstrom thick, appears on the siliconsubstrate 11 as shown in FIG. 3C.

[0061] Thereafter, through a known process, a flush-type nonvolatilememory, for example, is obtained.

[0062]FIG. 4 shows a configuration of the flush-type nonvolatile memoryobtained by the first and second embodiments. In FIG. 4, a plurality ofbits of nonvolatile memory transistors MT are formed in a central part,and selection transistors ST are formed on the opposite sides. In eachmemory transistor MT, numeral 6 denotes its floating gate, 21 denotesits control gate, and 23 denotes its source/drain. In each selectiontransistor ST, numeral 24 denotes its gate, and 23 denotes itssource/drain. Numeral 4(15) denotes a gate insulating film, and 25denotes an inter-layer insulating film. In this structure, the gateinsulating film 4 or 15 is the element obtained by the first or secondembodiment.

[0063]FIG. 5 shows a nitrogen profile in the nitrogen oxide film made bythe same embodiment, and illustrates changes in nitrogen concentrationwith distance from the surface of the nitrogen oxide film, combiningrelations between the 100 angstrom thick nitrogen oxide film 15 and theunderlying silicon substrate 11. It is apparent from FIG. 5 that thenitrogen concentration profile has two maximum peak positions, namely,the boundary face between the silicon substrate 11 and the nitrogenoxide film 15, and a position distant therefrom by approximately 80angstrom.

[0064] After that, following an ordinary flow, the process is forwardedto the step of stacking a Si film to be processed to floating gates anda silicide film and the step of making or processing electrodes.

[0065] The nitrogen oxide film made by the embodiment using NO or N₂Ogas can be improved in electric properties because the hydrogenconcentration in the film can be decreased as compared with that made byusing NH₃ gas. Further, since the process according to the embodimentdoes not require the thermal process indispensable when using NH₃ gas toremove hydrogen, it is possible to overcome problems such as loss ofnitrogen, which is remarked as a drawback of the process of removinghydrogen, fluctuation in threshold value of transistors due to diffusionof impurities into the silicon substrate, for example.

[0066] Additionally, although the instant embodiment stack the nitrogenoxide film 12, made in an initial step, to the thickness of 20 angstrom,and the silicon film 13 to 40 angstrom, the nitrogen oxide film 12,initially made, and the silicon film 13 may be changed in thickness tothereby finally obtain a desired thickness of the nitrogen oxide filmand locate the maximum peak of the nitrogen concentration profile at adesired position.

[0067] Furthermore, also in the second embodiment, nitrogen may bedoped. And, steps of the first embodiment may be combined with thesecond embodiment.

[0068] As described above, the manufacturing method of a semiconductordevice proposed by the invention can control nitrogen concentration inthe nitrogen oxide film to desirably locate its peak of peaks, and canprevent problems caused by a step of removing hydrogen by omitting thisstep, as compared with a process using NH₃ gas. Therefore, it iseffective in improving electric properties of the gate oxide film.

[0069] The drawings used for the foregoing explanation are made foreasier understanding of the invention, and dimensions thereof andelements therein are different from their actual sizes.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor substrate; a gate oxide film formed on said semiconductorsubstrate; and first transistors each having a first gate formed on saidgate oxide film and a pair of source/drain formed in confrontation insaid semiconductor substrate, said gate oxide film having a highernitrogen concentration in portions thereof nearer to said first gatesthan that of a portion thereof nearer to the semiconductor substrate. 2.The semiconductor device according to claim 1 wherein said gate oxidefilm has the maximum peak of nitrogen concentration distribution neareach said first gate, and concentration as said maximum peak isapproximately 1 through 5 atomic %.
 3. The semiconductor deviceaccording to claim 1 wherein each said first transistor is a nonvolatilememory transistor, and has a second gate formed above said first gatevia an insulating film.
 4. The semiconductor device according to claim 3wherein said semiconductor device is a NAND nonvolatile memoryincluding: an array of said first transistors arranged to commonly shareeach said source/drain between every two adjacent ones of said firsttransistors; and a second transistor arranged in one side and the otherside of said first transistors.
 5. The semiconductor device according toclaim 4 wherein each said second transistor has a gate made of said gateoxide film.
 6. The semiconductor device according to claim 2 whereineach said first transistor is a nonvolatile memory transistor, and asecond gate is provided above said first gates via an insulating film.7. The semiconductor device according to claim 6 wherein saidsemiconductor device is a NAND nonvolatile memory including: an array ofsaid first transistors arranged to commonly share each said source/drainbetween every two adjacent ones of said first transistors; and a secondtransistor arranged in one side and the other side of said firsttransistors.
 8. The semiconductor device according to claim 7 whereinsaid second transistor has a gate made of said gate oxide film.
 9. Amethod for manufacturing a semiconductor device including firsttransistors each having a first gate on a gate insulating film on asemiconductor substrate, in which said gate insulating film isfabricated by a process comprising: a first step of making a first oxidefilm on said semiconductor substrate; a second step of making a siliconfilm on said first oxide film; a third step of making a first nitrogenoxide film on said silicon film; a fourth step of segregating nitrogenalong a surface of said first nitrogen oxide film; and a fifth step ofoxidizing said silicon film to form a second nitrogen oxide film fromsaid first oxide film, said silicon film and said first nitrogen oxidefilm.
 10. The method according to claim 9 wherein said third to fifthsteps are conducted by introducing a gas containing nitrogen under astate with a high temperature and a high pressure.
 11. The methodaccording to claim 9 wherein, in said fourth step, the maximum peakconcentration of nitrogen concentration is made along said surface ofsaid first nitrogen oxide film.
 12. The method according to claim 11wherein said first transistor is a nonvolatile memory cell, and saidprocess further including a step of making a second gate above the firstgate of each said first transistor via an insulating film.
 13. Themethod according to claim 11 wherein said first transistor is anonvolatile memory cell, and said process further including a step ofmaking selection transistors at opposite sides of said firsttransistors.
 14. A method for manufacturing a semiconductor deviceincluding first transistors each having a first gate on a gateinsulating film on a semiconductor substrate, in which said gateinsulating film is fabricated by a process comprising: a first step ofmaking a first nitrogen oxide film on said semiconductor substrate; asecond step of making a silicon film on said first nitrogen oxide film;a third step of making a second nitrogen oxide film on said siliconfilm; a fourth step of segregating nitrogen along a bottom surface ofsaid first nitrogen oxide film and along a top surface of said secondnitrogen oxide film; and a fifth step of oxidizing said silicon film toform a third nitrogen oxide film from said first nitrogen oxide film,said silicon film and said second nitrogen oxide film.
 15. The methodaccording to claim 14 wherein said third to fifth steps are conducted byintroducing a gas containing nitorgen under a state with a hightemperature and a high pressure.
 16. The method according to claim 14wherein, in said fourth step, the maximum peak concentration of nitrogenconcentration is made along said bottom surface of the first nitrogenoxide film and along said top surface of the second nitrogen oxide film.17. The method according to claim 16 wherein said first transistor is anonvolatile memory cell, and said process further including a step ofmaking a second gate above the first gate of each said first transistorvia an insulating film.
 18. The method according to claim 17 whereinsaid first transistor is a nonvolatile memory cell, and said processfurther including a step of making selection transistors at oppositesides of said first transistors.